Recently, LEDs (Light Emitting Diodes) that need little electricity have been examined in use for a turning signal lamp of a vehicle instead of an electric bulb.
With respect to the electric bulb used in the turning signal lamp, as electric current starts flowing through a filament of the bulb, an electric bulb increases its luminance with increasing filament temperature, as shown in FIG. 7. As electric current conversely stops flowing through the filament of the bulb, the electric bulb decreases its luminance with decreasing filament temperature, as shown in FIG. 8. The electric bulb has thus a nonlinear characteristic of luminance variation. Further, a start-up period (or lighting-up period) during which luminance of the bulb becomes stable is approximately 300 ms, while a falling period (or lighting-out period) during which luminance of the bulb becomes zero is approximately 100 ms. Luminous intensity of the turning signal lamp using the electric bulb thereby varies with a slow response characteristic.
In contrast, with respect to LEDs, both start-up period and falling period are not more than 1 μs. Luminous intensity of the turning signal lamp using the LEDs thereby varies with a quick response characteristic.
Therefore, it has been proposed that a control circuit of LEDs has a slow luminance variation during its lighting-up period or lighting-out period (See JP-2001-244087A). Here, the luminance of the LEDs is gradually varied by varying a duty ratio of electric current flowing through the LEDs during the light-up and light-out periods.
In this control circuit, when the duty ratio is linearly increased during a lighting-up period, luminance of LEDs linearly increases, as shown in FIG. 9. When the duty ratio is conversely linearly decreased during a lighting-out period, the luminance of LEDs linearly decreases, as shown in FIG. 10. Linear varying of the duty ratio thus leads to gradual variation in a luminance characteristic of the LEDs. However, this luminance variation characteristic of the LEDs differs from that of the electric bulb, results in offering feeling of strangeness to a user.